This invention relates to the art of forming powder images and, more particularly, to heat and pressure belt fusing.
In the art of xerography or other similar image reproducing arts, a latent electrostatic image is formed on a charge-retentive surface which may comprise a photoconductor which generally comprises a photoconductive insulating material adhered to a conductive backing. When the image is formed on a photoconductor, the photoconductor is first provided with a uniform charge after which it is exposed to a light image of an original document to be reproduced. The latent electrostatic images, thus formed, are rendered visible by applying any one of numerous pigmented resins specifically designed for this purpose.
It should be understood that for the purposes of the present invention the latent electrostatic image may be formed by means other than by the exposure of an electrostatically charged photosensitive member to a light image of an original document. For example, the latent electrostatic image may be generated from information electronically stored or generated, and this information in digital form may be converted to alphanumeric images by image generation electronics and optics. However, such image generation electronic and optic devices form no part of the present invention.
In the case of a reusable photoconductive surface, the pigmented resin, more commonly referred to as toner which forms the visible images is transferred to a substrate such as plain paper. After transfer the images are made to adhere to the substrate using a fuser apparatus. To date, the use of simultaneous heat and contact pressure for fusing toner images has been the most widely accepted commercially, the most common being ones that utilize a pair of pressure engaged rolls.
Heretofore, it has been necessary with the foregoing type of fuser to heat the fuser not only when images are being fused but also during standby when images are not being fused. This is because of the long delay of raising the large thermal mass up to fusing temperature if the heat supply was turned off during standby. Such delays would not be tolerated by the user even though operating the fuser in such a manner would eliminate a substantial waste of energy.
Elimination of fuser standby power has been accomplished in prior art devices such as flash fusers and cold pressure fusers. Both of these types of fusers, however, exhibit other drawbacks. For example, cold pressure fusers exhibit poor quality images and they significantly limit the process speed of imaging machines.
Flash fusers create undesirable effluents and they work very poorly with colored toners, especially the lighter colored ones. Also, the optical density of flash fused images is unsatisfactory. Like cold pressure fusers, flash fusers limit the process speed of the imaging machines in which they are used.
Belt fusers are exemplified by the following publications and patent applications:
U.S. patent application Ser. No. 08/168,835, filed on Dec. 16, 1993 which is assigned to the same assignee as the instant invention relates to a belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which a substrate carrying toner images passes with the toner images contacting fusing belt. Electrical power is applied to the three contact rolls in such a manner that only the portions of the belt in the fusing zone are heated. Thus, the energy is concentrated only in the part of the fusing belt where it is needed for fusing the toner images on the final substrate. Thus, the free extent of the belt or in other words the portion of the belt outside of the fusing zone remains unheated.
To ensure good electrical contact in the presence of silicone oil contamination on the electrically resistive inner surface of the fusing belt, the contact rollers are textured by knurling, bead blasting or other suitable means.
U.S. patent application Ser. No. 08/169,836, filed on Dec. 16, 1993, which is assigned to the same assignee as the instant invention relates to a belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which a substrate carrying toner images passes with the toner images contacting a fusing belt. Electrical power is applied to the three fuser rolls in such a manner that only the portions of the belt in the fusing zone are heated to a predetermined operating temperature in accordance with a setpoint temperature. The free extent of the belt or in other words the portion of the belt outside of the fusing zone is adapted to be heated to various operating temperatures in order to produce prints with different gloss as desired.
U.S. patent application Ser. No. 08/169,838, filed on Dec. 16, 1993, which is assigned to the same assignee as the instant invention relates to a power controller, which does not rely on the use of sensors such as thermistors to control the operating temperature of a belt fuser. It features various preset inputs to control: steady state watts/in, cold start boost watts/in, warmup and cooldown time constants.
The controller sets the desired power based on the on-off cycling of the system. There are no sensors used to measure fuser temperature. For a cold start, the steady state plus boost power is used, during warmup the boost level is exponentially decreased at a rate set by a warmup time constant. When at rest (with no applied power) the power setpoint is exponentially increased at a rate set by a cooldown time constant.
U.S. patent application Ser. No. 08/169,802, filed on Dec. 16, 1993, which is assigned to the same assignee as the instant invention relates to a belt fuser for fusing transparencies without having to resort to off-line methods and apparatus. The toner images which are formed on the transparency during the imaging process have time to cool prior to separation from a smooth-surfaced belt.
U.S. patent application Ser. No. 08/168,891, filed on Dec. 16, 1993, which is assigned to the same assignee as the instant invention relates to a belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone. Electrical power is applied to the three fuser rolls in such a manner that only the portions of the belt in the fusing zone are heated. Thus, the energy is concentrated only in the part of the fusing belt where it is needed for fusing the toner images a final substrate. The free extent of the belt or in other words the portion of the belt outside of the three rollers remains unheated. Toner images are directly formed on or transferred to the unheated portion of the fusing belt. The images carried by the belt are then moved through the fusing zone where the images are simultaneously fused and transferred to a final substrate.
U.S. Pat. No. 4,565,439 granted to Scott D. Reynolds on Jan. 21, 1986 relates to a belt fuser for fusing toner images. The fusing apparatus is characterized by the separation of the heat and pressure functions such that the heat and pressure are effected at different locations on a thin flexible belt forming the toner contacting surface. A pressure roll cooperates with a non-rotating mandrel to form a nip through which the belt and copy substrate pass simultaneously. The belt is heated such that by the time it passes through the nip its temperature together with the applied pressure is sufficient for fusing the toner images passing there through. The non-rotating mandrel is adapted to having its axis skewed relative to the axis of the pressure roll. A pair of edge sensors are provided for activating a mandrel skewing mechanism. Skewing of the mandrel by such mechanism effects proper belt tracking.
U.S. Pat. No. 4,563,073 granted to Scott D. Reynolds on Jan. 7, 1986 relates to a low mass heat and pressure fuser and release agent management system therefor.
U.S. Pat. No. 5,084,738 granted to Noriyoshi Ishikawa on Jan. 28, 1992 discloses a fusing apparatus having an electrically conductive film which moves in contact with a recording material to which a toner image has been transferred, a pressing roller for causing the film to be brought into contact with the recording material and a plurality of electrodes disposed along a nip between the film and the pressing roller at a position opposing this pressing roller. The electrically conductive film heats up substantially only in the nip as the result of an electrical conductance to this electrode. The toner image on the recording material is heated and fixed by the heat generated in the electrically conductive film positioned in the nip. In a modified embodiment of the foregoing fusing device, a fusing film is fabricated using a thin-film conductive layer made by aluminum deposition or the like. The conductive layer is disposed on the side of a base film comprising carbon black added to a polycarbonate that will contact the transfer material on which a picture image is carried. Power is supplied between a first electrode and a second electrode. Joule heat is produced in the thickness direction of the fusing film.
U.S. Pat. No. 5,182,606 granted on Jan. 26, 1993 discloses an image fusing apparatus including a heater; a film movable with a recording material, in which the recording material has a toner image thereon which is heated through the film by heat from the heater; and the film has a heat resistive resin base layer containing inorganic electrically insulative filler material and a parting layer containing electrically conductive filler material.
In a fusing arrangement similar to the one disclosed in the '738 patent but where an endless belt is substituted for the web heater element similar to the one disclosed in the '606 patent, resistive heating is generated in both the fusing zone and the belt free span. In a continuous or endless belt fusing system, depending on the ratio of fusing zone length to the total belt length, up to 50% of the power may be dissipated by the belt free span.